High level electrical discharges or arcs within a dynamoelectric machine produce radio frequency (RF) emissions which provide operators with a warning of impending failure when properly detected. The sources of such electrical activity include: (1) broken conductors in the rotor or stator windings, (2) intermittent strand or turn shorts in the rotor or stator winding and (3) occurrences of severe slot discharges. Various monitor devices and systems have been previously described.
U.S. Pat. No. 3,813,160 issued to Cronin et al, Aug. 20, 1974, discloses a monitoring system for detecting continuous voltages and currents on a rotating shaft. The current is measured by measuring the voltage drop across a shunt while the voltage is directly measured. When predetermined levels for either voltage or current are exceeded, an alarm sounds.
Sawada et al in U.S. Pat. No. 4,058,804 issued Nov. 15, 1977, disclose a signal monitoring system for monitoring generator slip ring brushes. Radio frequency emission from the slip ring brushes is detected and recurring noise signals are subtracted. The circuit actuates an alarm if the RF signal from the brushes exceeds a predetermined value.
U.S. Pat. No. 4,502,046 issued Feb. 26, 1985 to Wonn et al disclose a detection system for rubs between the rotating and stationary parts of a turbine by connecting the rotor to electrical ground and introducing electrical energy into the rotor grounding circuit. A rub between the rotor and the stationary part modifies the currents in the ground circuit to an extent which actuates an alarm. In another embodiment of patent '046, the rotor discharge current in one or more grounding devices normally associated with a turbine is monitored to detect any abnormal conditions.
Radio frequency (RF) emissions are readily detected by an RF monitor, and the diagnostic importance of radio frequency emission from power generators was reported by F. T. Emery and R. T. Harrold in an article entitled "On Line Incipent Arc Detection in Large Turbine Generator Stator Windings", IEEE Transactions on Power Apparatus and Systems, Vol. PAS-99, p. 2232, November/December, (1980). In field use reported here, the RF signals were detected by a high frequency current transformer which was coupled to the neutral ground connection of the generator.
U.S. Pat. No. 4,853,818 issued to Emery et al on Aug. 1, 1989, and assigned to the assignee of this application disclose the RF monitoring of a dynamoelectric machine by coupling a sensor to the neutral common ground. The output from a capacitative sensor is provided to an RF monitor for analysis and diagnostic use in the maintenance of the generator. The RF monitor does not distinguish between signals which are warnings of impending breakdown and signals which are routine emissions from one of several sources in the dynamoelectric machine. Such sources include (1) d.c. field slip rings, (2) shaft grounding brush and (3) isolated phase bus. The RF emissions from the normal sources produces false alarms at the RF monitor and thus seriously detract from the reliability and acceptability of the radio frequency monitor as a warning system.